Field-Scale Monitoring of Agricultural Ditches as Conduits of Nitrogen, Phosphorus, and Suspended Sediment in Response to Storm Events and Low-Input Drainage Management: A Case-study of the Tchula Lake Farm

Author(s): Pierce, S.; Kröger, R.; Prevost, D.; Pierce, T.

Runoff from row-crop agriculture is a major source of non-point source aquatic pollution. High concentrations of inorganic nitrogen and sediment-bound phosphorus that are conveyed in agricultural drainage ditches can lead to eutrophication of receiving waters at both local and regional scales. Concerns regarding accelerated eutrophication have led to a concerted effort toward understanding the movement of nutrients across the landscape and the management of agricultural drainages for water quality remediation. This study monitors field-scale movements of nitrate, nitrite, ammonium, dissolved phosphorus, particulate phosphorus and total suspended solids through agricultural ditches over several months preceding and following the implementation of controlled drainage practices including riser boards, slotted pipes, and low-grade weirs. Water samples were collected during baseflow conditions and storm events via grab sampling and different automated techniques. Pre-implementation data collected from January-July of 2011 is summarized below. Preliminary post-implementation data will be presented as it becomes available. Nitrate concentrations showed a high degree of variability both spatially and temporally, varying from approximately 0 to 15 ppm, with values higher in the watershed tending to be slightly higher than outflows. Storm events generally had nitrate concentrations 50% to 100% greater than baseflow concentrations. Nitrite also demonstrated temporal variation, with some samples approaching 1 ppm. Compared to nitrate, however, spatial differences and total variance was low. In contrast to oxidized nitrogen, ammonia concentrations generally ranged from 0.1 to 0.3 ppm regardless of time or location. Dissolved inorganic phosphorus concentrations ranged from approximately 0 to 1.5 ppm, varying nonlinearly with date the sample was collected, but did not appreciably change in response to storm events. Total inorganic phosphorus and turbidity approached an order of magnitude higher in stormwater samples than baseflow samples, with mean total inorganic phosphorus of less than 2 ppm in baseflow samples compared to mean values greater than 10 ppm in stormwater samples. Total suspended sediment concentrations were also significantly higher in stormwater samples than baseflow samples, indicating the likelihood that erosion or sediment resuspension is a major factor in phosphorus transport in agricultural drainage ditches. Preliminary data following implementation of controlled drainage will be compared with this background data set to determine short-term impacts of ditch reshaping on water quality and to estimate the effects of vegetation establishment.
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Technical Sessions

1 Best Management Practices #1
2 Delta Water Assessment
3 Flood Assessment & Mgmt.
4 Wetlands
5 Watershed Mgmt. #1
6 Non-Point Source Assessment
7 Modeling
8 Water Quality
9 Best Mgt. Practices #2
10 Delta Water Conservation
11 Sedimentation
12 Storm Water
13 Watershed Mgt. #2
14 Public Water Systems
15 Surface Water Assessment & Evaluation

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